soc: qcom: smem: fix first cache entry calculation

Cached items are found at the high end of an smem partition.  A
cached item's shared memory precedes the private entry structure
that describes it.

The address of the structure describing the first cached item should
be returned by phdr_to_first_cached_entry().  However the function
calculates the start address using the wrong structure size.

Fix this by computing the first item's entry structure address by
subtracting the size of a private entry structure rather than a
partition header structure.

Signed-off-by: Alex Elder <elder@linaro.org>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Andy Gross <andy.gross@linaro.org>